Up to now, the preferred standard method of interfacing a gas chromatograph to a mass spectrometer has been to insert the GC capillary directly into the MS source. Although it is inconvenient and only possible over a limited range, the MS source pressure may then be adjusted to acceptable levels for relatively high GC flows by adjusting the conductance of the source. This may be done by interchanging sources of different conductance, or by interchanging plugs or orifices in the opposite or unused sample channel of the MS source, or to a limited extent, by substituting a pump with greater speed or pumping capacity in the high vacuum system. A compromise must still be made between source pressure, analytical sensitivity and sample residence time. In general, residence time must be kept very short (less than one second) to preserve narrow capillary GC peaks. Opening the source keeps the pressure at acceptable levels and the residence time short but reduces sensitivity. The choices are much more limited with low pumping capacity mass spectrometers because of the effects of the high background manifold pressure.
With the present invention, the source pressure and the fraction of the GC effluent which flows directly to the source can be adjusted conveniently, accurately and in a continuously variable manner from zero to 100% by controlling the position of the end of the GC capillary in an interface attached to the source. In addition, considerable enrichment of the sample stream may be effected under certain experimental conditions which is not possible with a direct connection.
It should be noted that with any MS source, there must be an orifice or a short channel through which the MS capillary is inserted into the source up to some point near and perpendicular to the electron beam. It necessarily follows that there may be some slight variation in the source conductance and the flow split, depending on the exact position of the end of the column and the geometry of the channel, and on whether the opening is a channel or an orifice. This is incidental to the design and purpose of the standard source and does not compare to the present invention in which the entire inlet channel is itself an MS column which extends at all times, all the way up to the electron beam and may be calibrated very accurately for flow split.